The present invention relates generally to internal combustion engines and more particularly to expandable chamber piston engines operating in an open thermodynamic cycle.
Automotive vehicle and engine manufacturers, fuel injection equipment suppliers and, indeed, society as a whole, share in the desire for efficient, effective transportation. The balance between combustion processes to produce power, and those processes which create pollution, is best addressed by enhancing the fundamental efficiency of the engine processes.
It is well known that the ideal Carnot Cycle, in which isothermal heat addition and rejection are combined with isentropic compression and expansion, is the most efficient engine cycle for any given upper and lower operating temperatures. However, the Carnot cycle is not practical for an expanding chamber piston engine due to the very high (over 50:1) compression ratio required to produce significant power. Nevertheless, a practical process which could make some use of the highly efficient Carnot process would be an advance in the art.
The most practical engine, and thus presently the most predominant, is the Otto cycle engine which includes a compression process of a fuel-air mixture followed by unregulated combustion. It is well known that for a given compression ratio the ideal Otto cycle is the most efficient expanding chamber piston engine since the Otto cycle combines high peak temperature with a practical average temperature of heat input. However, the high peak combustion temperature of an Otto engine can cause auto-ignition of a portion of the fuel-air mixture, resulting in engine noise and damage to the engine, as well as the creation of excess amounts of undesired NOx.
In the past, auto-ignition in Otto cycle engines was reduced by use of chemical additives to the fuel such as lead compounds (no longer permitted by law), manganese compounds (which cause spark plug deposits to build up, resulting in misfire), benzene (the use of which is presently being curtailed by legislative mandate) or fuel reformulations to prevent deleterious auto-ignition while meeting environmental goals. Auto-ignition can also be reduced by limiting the combustion temperature, either through use of a lower compression ratio (which reduces both power and efficiency), or by exhaust gas recirculation, lean-burn or stratified charge techniques, all of which cause power loss.
For general purpose road use, the engines of emission-constrained passenger cars are presently limited to useful compression of about 10:1. Above that limit the increased cost of the fuel control system and the additional cost of more platinum or rhodium for exhaust catalytic converters generally outweighs the benefit of higher compression ratios. A technology which would allow a practical Otto compression process to operate at compression ratios higher than 10:1 would be an advance in the art.
An improvement on the Otto cycle, as represented by a higher useful compression ratio, is an ideal Diesel cycle comprising isothermal heat addition and isochoric (constant volume) heat rejection combined with isentropic compression and expansion. This ideal Diesel cycle overcomes the fuel octane limit of the Otto cycle by utilizing air alone for the compression process and mixing the fuel with the process air as part of the combustion process. This allows use of a low octane-rated fuel, but requires cetane-rated fuel (enhanced auto-ignition). However, the isothermal process of the aforedescribed ideal Diesel cycle was found to be impractical, due to the extremely high compression ratio (50:1) required, and an alternate heat addition process (isobaric or constant pressure) was put into practice.
Another variation on the ideal Diesel cycle is the ideal limited pressure cycle including combined isochoric and isobaric heat addition, and isochoric heat rejection combined with isentropic compression and expansion. This combustion process allows an engine to be operated at moderate compression ratios (14:1 to 17:1 for large open chamber engines) as well as high compression ratios (20:1 to 25:1 for small displacement engines).
While Diesel-type engines are fuel efficient, due to their high compression ratio, they tend to be heavier and lower in power than an Otto engine of the same displacement. In addition, all direct injection engines of the Diesel type suffer from an ignition lag which reduces the control and effectiveness of the combustion process. One way to overcome this ignition lag is to preheat the fuel to 1,500xc2x0 R before injection. This produces hypergolic combustion upon injection, but is an impractical method due to the short service life of the injector nozzle.
Hybrid engine processes have been developed incorporating characteristics of both diesel and spark ignition engines but these have proven impractical for road use. Examples of these hybrid processes include the Texaco TCCS, the Ford PROCO, Ricardo, MAN-FM and the KHD-AD. All employ open chamber, direct injection spark ignition engines using stratified charge techniques to improve efficiency. These developmental engines suffer substantial power loss because of ignition lag, incomplete utilization of the process air and poor mixing of the fuel/air charge.
Because the limits of current technology are thus being reached, there exists a need for an internal combustion engine that will provide a better balance between power production, fuel efficiency, pollution creation and pollution control by use of a more practical combination of thermodynamic processes.
Basically, the present invention meets the foregoing requirements and constraints by utilizing a new combination of thermodynamic processes which limits maximum combustion temperatures, thereby enabling an internal combustion engine to operate at a higher compression ratio, a higher power output or a lower peak temperature with a given fuel.
Broadly, in accordance with one exemplary embodiment, the invention is practiced by controlling the fuel quantity and injection timing of a direct injection system in an internal combustion engine, so as to produce a combustion process consisting of a constant volume (isochoric) phase and a constant temperature (isothermal) phase. The limited temperature engine cycle so achieved allows the use of substantially higher compression ratios with a given fuel or with a given NOx emission limit, thereby providing a higher practical thermal efficiency than the standard lower compression ratio Otto cycle when measured by fuel/air analysis or by analyzing the test data of an actual engine.
In addition, the limited temperature cycle so achieved allows a higher power output and a lower NOx creation rate at a given compression ratio with a low quality fuel.
In accordance with another aspect of the invention, there is provided a new method of operating an expanding chamber internal combustion piston engine for providing limited temperature combustion. Such an engine includes at least one cylinder and an associated piston for forming a combustion chamber with the piston having a top dead center position; an operating cycle including an intake stroke, a compression stroke and an expansion stroke; and a fuel introduction system. The method of operating the engine pursuant to the invention comprises the steps of first forming a predetermined fuel/air mixture by introducing a predetermined fraction in one or more discrete quantities of the total fuel necessary for complete combustion of the process air. Next, the relatively lean fuel/air mixture so introduced is ignited when the piston is substantially at top dead center, this first phase of combustion thereby comprising a substantially isochoric or constant volume process. The fuel supplied for the isochoric process is an amount which will produce a greatly reduced temperature of the working fluid, as low as 3,300 degrees Rankine, or less, even at high compression ratios. Last, there is introduced, substantially at the beginning of the expansion stroke, a second fraction (in one or more discrete quantities) of the total fuel necessary for complete combustion. The combustion resulting from the introduction of the second fraction is a substantially isothermal process. The isothermal process occurs at a temperature which is significantly less than that attained in a comparable Otto cycle engine having the same or a substantially lower compression ratio. NOX emissions are thereby limited and such reduction is obtained at lower cost than existing systems.
Those skilled in the art will recognize that the method of the present invention makes use of the Otto process for the first phase of the heat input or combustion process and the Carnot process for the second phase of heat input or combustion process. Comparison of the operating cycle of the invention with the standard Otto cycle using ideal fuel/air analysis shows an unexpected benefit from the invention: the overall operating efficiency of an engine (with a given compression ratio) will be greater using the limited temperature cycle of the present invention than when using the Otto cycle, when high temperature losses are considered. This increase in efficiency at a given compression ratio is a benefit derived from reduced cycle temperature.
Another advantage of the present invention is that it allows an engine to be operated more efficiently (at a higher compression ratio) than is possible with present engines. The most readily available motor vehicle gasoline fuels have combustion quality ratings of about 90 octane, which generally limits many engines to a compression ratio of about 10:1 for public road use. Since octane rating is indirectly related to high combustion temperature (high operating temperatures require high octane fuel), and the invention reduces the operating temperature, it follows that the invention enables the use of a higher engine compression ratio with a commensurate gain in engine efficiency.
In sum, the method of the present invention allows a practical engine to make use of an ideal process: during the isothermal combustion process, heat energy is converted directly to work. The invention utilizes present engine design and materials and may be practiced by modifying existing internal combustion engines to incorporate the desired compression ratio and appropriate fuel introduction systems.